This invention relates to television synchronizing apparatus and especially to apparatus for producing the Bruch blanking waveform necessary in any
sync generating or regenerating apparatus.
The television broadcast systems of many countries and closed circuit operations utilize the PAL standards as defined by the CIRR standards, XII Plenary Assembly, 1970, Vol. V, Part 2, Report No. 407-1. These standards specify that the R-Y phase of chroma be inverted every television line. The burst signal on each line therefore alternates in phase between +135.degree. and -135.degree. from the B-Y reference. A further requirement of all PAL systems is that during the vertical interval when burst is suppressed, the first and last burst signals must be in the +135.degree. phase. This latter requirement requires that the burst blanking signal must be staggered with respect to a vertical interval reference in any odd-line Pal system, such as the 525 or 625 lines-per-frame systems. This staggered burst signal blanking sequence, which is repeated once every four fields, is commonly referred to as the meandering burst blanking signal or in still shorter terms as Bruch blanking.
Signals in the form of pulses that repeat their cycle once per field are commonly available in any sync generating or regenerating circuitry, or they may be extracted from the vertical interval by well-known techniques and delayed to occur at any time with respect to the beginning of vertical sync. This known signal may be applied directly to a burst blanking circuit for an NTSC television system where the burst phase is the same for every line, and burst blanking is the same for every field. The burst blanking cycle for PAL systems, however, is more complex due to the unique four-field sequence utilized.
In known circuits, the necessary Bruch blanking sequence for a 625 line television system is developed in the following manner. A 625 line system requires a burst blanking interval of a duration of nine television lines (9H) with the position of this 9H blanking period to be staggered within successive fields. The burst blanking interval plus the burst pulse train must always comprise 2n lines because the burst phase alternation, as well as the PAL square wave switching signal for the R-Y and B-Y decoders, cannot be interrupted. Therefore, it follows that three of the four fields of a full cycle must contain an uninterrupted burst train of 303 lines duration and the fourth field requires a burst train of 304 lines. These unique burst trains, each beginning and ending with a different relationship to the known vertical interval sync pulse, are developed by extracting various pulses from the sync signal and utilizing them in numerical counting circuits. Typically, a first counter circuit actuated by the PAL square wave determines the stop of the 9H blanking interval, while a second counter counts the intermediate square wave cycles which have occurred since the end of the preceeding field. A suitable delay of the vertical sync interval pulse modifies the total burst train count to determine the start of the blanking period; for example, the second counter of 154 stages starts counting with a one-line delay in the second, third and fourth fields, but with a three-line delay in the first field, thus providing a total burst train sequence count of 2(154-3) + 1 = 303 lines in the second, third and fourth fields and 2(154-3) + 3 = 305 lines in the first fields. A commercially available form of the above-described circuitry is illustrated in the TR-70 video tape recorder manufactured by RCA Corporation, Camden, New Jersey, United States of America. In this apparatus, a 9H stop generator is gated from the horizontal line rate, and a counter circuit of 155 counts is modified for each field by a decoding circuit which combines vertical sync pulse with the vertical equalizing pulses of each field to provide the proper line count for each of the unique fields.
These known circuit arrangements, which employ high numerical counters and complex gating or decoding circuits, do not provide for flexibility of operation, particularly when equipment, such as video tape recorders and cameras, are required to operate between one or more standards, for example, PAL 625 to PAL 525 line standards. Such a change requires a reprogramming of the blanking interval, counter circuit and line count correcting circuitry. Since these known circuits depend on the counter and delay gating circuits having been set correctly during the previous vertical interval, interruptions of the television signal, such as may occur during a drop-out in a video tape recorder or signal fading in a video process, will result in a loss of proper blanking sequence. Similarly, when video tape recordings are edited and signals on a frame-to-frame basis are intermixed, extreme care is required to preserve or minimize the loss of the proper blanking sequence.
The present invention provides a unique means of providing Bruch blanking without the use of high numerical counters and complex gating circuitry. A single pre-vertical sync pulse that repeats once each field and the line-by-line chroma phase information which exists in the form of the PAL square wave at one-half of the horizontal line frequency are combined in a novel circuit comprising a pair of J-K flip-flops to directly produce the complete Bruch blanking function in digital form. The single pre-vertical pulse modified by line-to-line information contained in the PAL square wave provides the correctly phased Bruch blanking interval for each field on an independent field-by-field basis without depending on the previously mentioned counting circuits and gating delays having been set correctly during the previous vertical interval.